The present invention relates to a system for controlling an oxygen concentration in exhaust gases emitted from a combustion engine, and more particularly it relates to an improvement in which a biasing current supplied to a resistive type oxygen sensor used to control an oxygen concentration in exhaust gases is varied to keep a sensor output voltage indicative of either presence or absence of oxygen at a predetermined magnitude.
In an engine system in which air-fuel mixtures are supplied to a combustion engine and combustion resultant exhaust gases are emitted from the combustion engine, it is required that the exhaust gases should include less noxious components such as HC, CO and NOx. For this requirement it is suggested that a catalyst effective to reduce the noxious components in the exhaust gases is provided in the exhaust passage. The catalyst presents a high efficiency in reducing the noxious components, when an oxygen concentration in the exhaust gases is kept at a constant value intermediate between a presence and absence of oxygen. Therefore, an air-fuel ratio of the mixture supplied to the combustion engine must be kept substantially at a stoichiometric value (air excess number .lambda.=1). Alternatively, when the air-fuel ratio of the mixture is below the stoichiometric value, that is, when the mixture is rich in fuel, the exhaust gases must be supplied with air at the upstream of the catalyst. It is most effective to feedback control the air-fuel ratio of mixture supplied to the combustion engine or the amount of air supplied to the exhaust gases in response to the oxygen concentration which may be sensed by an oxygen responsive element.
As the oxygen responsive element, a zirconium dioxide (ZrO.sub.2) and a titanium dioxide (TiO.sub.2) which exhibit changes in an electromotive force and an resistance respectively in response to the oxygen concentration are known. But such an oxygen responsive element also exhibits changes in the electromotive force or the resistance in response to ambient conditions such as aging and temperature. Therefore, the output changes of the oxygen responsive element resulting from ambient conditions other than the oxygen concentration must be reduced so that the air-fuel ratio of mixture supplied to the combustion engine or the amount of air supplied to the exhaust gases is feedback controlled in response to the output of the oxygen responsive element.